int rangeSumBST(struct TreeNode* root, int L, int R){
    if (root == NULL) {
        return 0;
    } else if (root->val >= L && root->val <= R) {
        return root->val + rangeSumBST(root->left, L, R) + rangeSumBST(root->right, L, R);
    } else {
        return rangeSumBST(root->left, L, R) + rangeSumBST(root->right, L, R);
    }
}
